1. Field of the Invention
The present invention relates to a motor-driven injection molding machine using a drive motor (servomotor) serving as a drive source.
2. Description of the Relevant Art
Generally, a motor-driven injection molding machine using a servomotor as a drive source potentially involves malfunction or breakage of the servomotor due to overheat arising under certain molding conditions or under a certain environment, since the servomotor generates heat according to load current. Therefore, the motor-driven injection molding machine usually employs measures for protection of the servomotor. Specifically, a heat generation value of the servomotor is detected. When the detected heat generation value reaches a preset stopping point, the value is interpreted as indicating overload, and the servomotor is forcibly stopped to thereby protect the servomotor.
For an injection molding machine, stoppage of a servomotor during molding means interruption of a molding process, which may have an adverse effect on the molding process once resumed. Therefore, interruption of a molding process must be avoided to the extent possible. In order to meet the end, for example, Japanese Patent Application Laid-Open (kokai) No. 11(1999)-235743 proposes a method for checking preset operating conditions. According to the proposed method, the injection molding machine is operated on a trial basis in order to judge, from load current flowing through a servomotor, whether or not the servomotor operates in an overloaded unstable region. When the servomotor operates in the overloaded unstable region, the preset operating conditions are reviewed so as to impart appropriate allowance thereto, to thereby avoid interruption of operation (molding process) during actual molding.
The above-mentioned conventional method involves the following problems.
First, since operating conditions are reviewed merely at the stage of trial operation, the review fails to reflect operating conditions during actual molding. Thus, interruption of molding cannot be reliably avoided; i.e., reliability in avoidance of operational interruption is rather poor.
Second, since the preset operating conditions are reviewed at the stage of trial operation so as to impart appropriate allowance thereto, the injection molding machine may fail to operate under optimum operating conditions, resulting in impaired molding quality or productivity.
An object of the present invention is to provide a method for controlling a motor-driven injection molding machine capable of reliably avoiding interruption of molding caused by merely transient overload, to thereby improve reliability.
Another object of the present invention is to provide a method for controlling a motor-driven injection molding machine capable of always conducting molding under optimum molding conditions to thereby avoid impairment in molding quality and productivity.
To achieve the above objects, the present invention provides a method for controlling a motor-driven injection molding machine which includes a plurality of drive motors. In the method, load current flowing through at least one drive motor serving as a drive source is detected in order to estimate heat generation value of the drive motor. When the estimated heat generation value reaches a preset stopping point, the drive motor is stopped. When the estimated heat generation value reaches a preset alarming point, which is lower than the stopping point, an alarm is issued.